libfuzzer: Document REDUCE event outputs

[chfast]

Bugzilla Link	43226
Version	unspecified
OS	Linux

Extended Description

There are some information missing from the libfuzzer documentation about the "L:" and "REDUCE" event. See https://llvm.org/docs/LibFuzzer.html#output section.

1. The example is outdated.
2. The "L:" column now has 2 values (e.g. "L: 17214/25000"). I'm guessing the second one is the length limit, but this is also provided in the "lim:" column?
3. The REDUCE event also outputs "L:" and "MS:" values, but this is undocumented. I would especially want to know that "L:" means in case of REDUCE.
4. The REDUCE sometimes adds a new file to corpus (the file count increases). Can you document why is this happening?